A photolithography technique is used for forming a circuit pattern on a substrate such as, for example, a semiconductor wafer. After a desired circuit pattern is formed, a resist film is removed from the substrate by using a sulfuric acid hydrogen peroxide mixture (SPM) liquid. The resist removal processing is performed, for example, by immersing a plurality of (e.g., fifty (50)) substrates in the SPM liquid stored in a processing bath while the substrates are held by a substrate holder called a wafer boat. The SPM liquid contains Caro's acid which is produced when sulfuric acid and hydrogen peroxide are reacted with each other, and the resist film is removed by being oxidized with Caro's acid (see, e.g., Japanese Patent Laid-Open Publication No. 2011-134899).
A substrate processing apparatus described in Japanese Patent Laid-Open Publication No. 2011-134899 includes a circulation system including a processing bath that stores an SPM liquid, an outer bath that receives the SPM liquid overflowing from the processing bath, and a circulation line connected to nozzles provided in the outer bath and the processing bath. In the circulation line, a pump that forms a circulation flow of the SPM liquid in the circulation system, a heater that heats the SPM liquid, and a filter that removes solid impurities in the SPM liquid, are interposed in this order from the upstream side. A hydrogen peroxide supply line for replenishing the circulation system with hydrogen peroxide is connected to an upstream position of a nozzle close to a nozzle of the circulation line.
In a batch type substrate processing apparatus similar to that described in Japanese Patent Laid-Open Publication No. 2011-134899, the Caro's acid is produced in a concentration and an amount sufficient to remove the resist film from the substrate. Thus, when the resist film is removed from the substrate, the resist film removed from the substrate may not be completely dissolved in the SPM liquid, so that the resist film is dispersed in a form of small pieces in the SPM liquid. When the small pieces of the undissolved resist film flow into a device such as, for example, the pump or the filter provided in the circulation line, the small pieces may attach to the device and adversely affect the device.